Unstationnary control of a launcher using observer-based structures

This paper deals with the design of a gain-scheduled controller for the attitude control of a launcher during atmospheric flight. The design is characterized by classical requirements such as phase/gain margins and flexible mode attenuations as well as time-domain constraints on the response of angle of attack to a worstcase wind profile. Moreover, these requirements must be fulfilled over the full atmospheric flight envelope and must be robust against parametric uncertainties. In order to achieve this goal, we propose a method based on minimal observer-based realizations of arbitrary stabilizing compensators. An original technique to assign the closed-loop dynamics between the state-feedback dynamics and the state-estimation dynamics is presented for the H∞ compensators case. The structure is used to mix various specifications through the Cross Standard Form(CSF) and to perform a smooth gain scheduling interpolation through an Euler-Newton algorithm of continuation

De l'utilisation de la structure estimation/commande pour le pilotage instationnaire d'un lanceur spatial

Dans cet article, nous nous intéressons au développement de techniques ayant trait à la mise sous forme LQG équivalente de correcteurs stabilisants. Dans un premier temps nous nous concentrerons plus particulièrement sur la transformation de correcteurs issus d'une synthèse Hinfinty. Nous proposons une méthode originale pour effectuer le choix des dynamiques d'estimation et de commande du correcteur équivalent en s'appuyant sur les résultats d'une synthèse H2. Puis nous proposerons une définition en temps discret de la Forme Standard de Passage (CSF) qui s'appuie également sur la paramétrisation de Youla sur la structure estimation /commande. Nous la présenterons en tant qu'outil méthodologique pour la mise en forme d'un problème multi-objectif. Nous mettrons en application la CSF sur le problème de pilotage d'un lanceur et nous montrerons l'intérêt de l'utilisation de la structure estimation/commande dans le cadre de l'inter-polation de correcteur pour le pilotage instationnaire

Launcher attitude control: some additional design and optimization tools

This paper deals with the launcher attitude control during atmospheric flight. A two step approach combining an H1 control design and an optimization procedure is proposed. The first step is multi-objective stationary H1 design based on the Cross Standard Form. It provides easily a first rough solution from a few physical tuning parameters. The second step is a fine tuning using an multi-constraint satisfaction algorithm. This algorithm enables the certification criteria computed on the validation model to be met and is also used to propagate the nominal tuning to the full flight envelope

On the Papaloizou-Pringle instability in tidal disruption events

We demonstrate that the compact, thick disc formed in a tidal disruption event may be unstable to non-axisymmetric perturbations in the form of the Papaloizou-Pringle instability. We show this can lead to rapid redistribution of angular momentum that can be parameterised in terms of an effective Shakura-Sunyaev $\alpha$ parameter. For remnants that have initially weak magnetic fields, this may be responsible for driving mass accretion prior to the onset of the magneto-rotational instability. For tidal disruptions around a $10^6$ M$_{\odot}$ black hole, the measured accretion rate is super-Eddington but is not sustainable over many orbits. We thus identify a method by which the torus formed in tidal disruption event may be significantly accreted before the magneto-rotational instability is established.Comment: 9 pages, 10 figures, accepted for publication in MNRAS. Movies of simulations available at https://youtu.be/kBLAjY8n9vI and https://youtu.be/F8F0tmLbX3

Launcher attitude control: discrete-time robust design and gain-scheduling

In this paper, a robust multi-objective design for the control of a launcher during atmospheric flight is investigated. This approach is based on the Cross Standard Form formulation which allows to incorporate the various specifications of the launcher problem in a streamlined manner. An important feature of this approach is that a non-conventional LQG/LTR approach, required to satisfy time-domain specifications, can be embedded into a more general standard problem in order to account for frequency-domain robustness constraints. The specific form of this standard problem is also very interesting for gain scheduling

Existence of the harmonic measure for random walks on graphs and in random environments

We give a sufficient condition for the existence of the harmonic measure from infinity of transient random walks on weighted graphs. In particular, this condition is verified by the random conductance model on $\Z^d$, $d\geq 3$, when the conductances are i.i.d. and the bonds with positive conductance percolate. The harmonic measure from infinity also exists for random walks on supercritical clusters of $\Z^2$. This is proved using results of Barlow (2004).Comment: 25 p. and 2 figure

Dark Optical Lattice of Ring Traps for Cold Atoms

We propose a new geometry of optical lattice for cold atoms, namely a lattice made of a 1D stack of dark ring traps. It is obtained through the interference pattern of a standard Gaussian beam with a counter-propagating hollow beam obtained using a setup with two conical lenses. The traps of the resulting lattice are characterized by a high confinement and a filling rate much larger than unity, even if loaded with cold atoms from a MOT. We have implemented this system experimentally, and obtained a lattice of ring traps populated with typically 40 atoms per site with a life time of 30 ms. Applications in statistical physics, quantum computing and Bose-Einstein condensate dynamics are conceivable.Comment: 4 pages, submitted to PR